First, explain
CentOS 6.8 kernel are generally 2.6, but the premise is to install docker CentOS kernel must be 3.10 or above, it will generally choose to upgrade to a later version, this upgrade version 4.4.186-1.el6.elrepo .x86_64.
Second, kernel upgrade
a view kernel command (the uname -R & lt) 
2, introduced Key
#rpm --import https://www.elrepo.org/RPM-GPG-KEY-elrepo.org
if error occurs as follows, nss indicates (network security service, network security services) is not installed or updated
curl: (35) Connect error SSL
error: https://www.elrepo.org/RPM-GPG-KEY-elrepo.org : Import the Read failed The (2) .
installation nss execute:
# yum install nss
update nss execute:
# yum update nss
3, after install or update nss rerun
# Rpm---import https://www.elrepo.org/RPM-GPG-KEY-elrepo.org
4, the mounting ELRepo the CentOS
#rpm -Uvh http://www.elrepo.org/elrepo-release-6-8.el6.elrepo.noarch.rpm
. 5, the core mounting
#yum --enablerepo = elrepo-kernel install kernel -lt -y
6, modify the boot file, the default boot to 0
#sed. 1 -is # default = 0 # default # G = the /etc/grub.conf
7, reboot the system core edition again
Third, the installation and start docker
#yum the install docker -Y-IO
#service docker Start